If you have 1000 listings, each with 4 images and in media settings you have 4 image sizes, you will create 16k useless posts in your database making your db much bigger in size and slower.
I had been wondering the reason behind not using the standard WP Media system.
However there are three disadvantages with the method you have implemented:
It makes it much more difficult to import from an existing web site, particularly a WP one, which is a major issue where the listings are in the thousands. Our gallery had been largely based on unique directories and the file names could be duplicated.
GeoDirectory should really be using at least two image sizes so that the most suitable sized image could be served. This would reduce the data transmitted for the pages where there are many listing on one page and possibly the detail pages on mobile. Google ranks pages based on these criteria, so it isn’t just an academic issue.
Loss of captions. GeoDirectory doesn’t have the option to show captions with pictures. Captions are useful and important for several reasons: It helps with SEO; it explains what is in the picture and it makes a big problem for the recording and displaying acknowledgement and copyright for a picture. Not a problem when it is our media, but a big problem when using pictures which require acknowledgement as part of their terms of use.
It would be relatively easy to add a field for captions in the standard GeoDirectory, without the need to use the standard WP media system.